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beam size. In such a situation, the beam lifetime r is given
by:
2 a 2 2 2
2 2 \6x -R ) \6Y -R) (1)
b
where a is a constant, b is the current per bunch, q. and
6y are the measured horizontal and vertical rms beam
sizes respectively and (m is the resolution of the
measuring system. In deriving Equation (1) two
assumptions were made: i) the resolution is the same in
both vertical and horizontal planes and ii) the bunch
length is constant at the currents used during the
measurement.
By changing the beam size (using skew quadrupoles for
example) and simultaneously measuring current, beam
lifetime and horizontal and vertical sizes, it is possible to
derive the system resolution using equation (1) for fitting
the data.
It is important that during this measurement, the overall
momentum acceptance of the ring must remain constant.
For example, changing the beam size using skew
quadrupoles could modify the dynamic aperture of the
machine. If the momentum acceptance is dominated by
the dynamic aperture term, the accuracy of the resolution
measurement could suffer. A simple way to avoid this
problem is to reduce the RF power enough that the
momentum acceptance is totally defined by the RF bucket
size.
1.2
x10 ' ' '
FIT: = a*(62- 6)(a - ,2 )/1b2
1.0 -6R = 33.37 pm
a=77.4 mA2s2 sm-4m
O.8-
0.6
0.4
0 1 2 3 4 5
Measurement Number
Figure 5: BL 7.2 resolution measurement.
The ALS is a Toushek dominated storage ring and
Figure 5 shows an example of this measurement
technique applied to BL 7.2 with the filter in the position
2 of Table 2. The measured resolution (~ 33 m) is
significantly larger that the design one (~ 24 m).
Additional measurements showed a similar situation for
the other filter positions with the best resolution value
measured at ~ 30 gm.
As mentioned in the introduction, BL 7.2 was designed
for measuring the ~ 100 m horizontal beam size during
the emittance-energy spread measurement. A resolution of
30 m adds in quadrature with the actual beam size giving
a few percent overestimated measurement. Such a small
error is easily removed by de-convolving the resolution
effect and will not affect the accuracy of the emittance-
momentum spread measurements. On the other hand we
wanted to understand the reason for the problem, so we
began a series of dedicated measurements and
investigations. First, we checked our resolution formulae
in [1] by means of the SRW code [4] and no inconsistency
was found. Pinhole dilation effects were totally negligible
and vibration measurements on different parts of the
beamline showed effects smaller than 1 m. Finally, we
measured the diameter of several pinholes on spare
pinhole matrixes. Two sets of independent measurements,
using an electron microscope and a transmitted light
device, showed diameters significantly larger than the
design goal. Such differences in the diameters are
consistent with the larger resolution values we measured.
The holes were drilled in a 150 m thick tungsten plate by
laser ablation. At the present time, we are investigating
the possibilities of having a better quality pinhole matrix
made by the same technique, or of moving towards a
different drilling technology.
CONCLUSIONS AND FUTURE PLANS
The x-ray pinhole camera system of BL 7.2, the new
diagnostic beamline at the ALS, has been successfully
commissioned and is now in routine operation. All the
design parameters were achieved with only the exception
of the system resolution, which is larger than expected.
The reason for this discrepancy has been understood and
the problem will be fixed in the near future. Even with the
larger resolution the beamline can accurately perform all
the measurements it was designed for.
Plans for the future also include the completion of the
second photon line with the x-ray BPM system and the
IR-visible port.
ACKNOWLEDGEMENTS
We want to thank J. Krupnick and D. Robin for the
continuous support and C. Steier and the other members
of ALS Accelerator Physics Group for the fruitful
conversations. Finally, we acknowledge the contributions
of B. Bailey, R. Duarte, S. Jacobson, V. Moroz and the
entire installation team.
REFERENCES
[1] F. Sannibale et al., "A second beam-diagnostic
beamline for the advanced light source", PAC 2003,
Portland, OR USA, May 12-16, 2003.
[2] F. Sannibale, "Experimental Error Analysis of a
Possible Measurement of the Emittance and
Momentum Spread at the ALS" ALS Note LSAP-301
(2003).
[3] W. B. Peatman and K. Holldack, "Diagnostic front
end for BESSY I", J. Synchrotron Rad. 5, 639 (1998).
[4] O. Chubar and P.Ellaume, Synchrotron Radiation
Workshop (SRW) code, http://www.esff.fr/machine
/groups/insertiondevices/Codes/software.html.
+
+ Measured
x Fit
+
x
Attn. Filter 3.97
